In general, a brushless motor has armature windings for three phases. A motor drive control apparatus supplies an arbitrary driving current to each phase of the motor through a power inverter circuit including a voltage-type inverter or the like. The motor drive control apparatus can arbitrary control torque by controlling this driving current with a current controller.
The motor drive control apparatus can rotate the motor at arbitrary speed by adding a speed controller at a front side of the current controller. The motor drive control apparatus can stop the motor at an arbitrary rotation position by adding a position controller.
In general, a CPU is used for control-arithmetic-processing in the current controller, the speed controller, and the position controller. In particular, the current controller converts using an AD converter voltage values of motor driving currents of three phases of inverter output, which are converted by a motor-driving-current detecting circuit, into numerical values. The current controller captures the numerical values into an operator such as the CPU, and performs a control operation of current control.
The motor-driving-current detecting circuit adopts a configuration for, with a resistor interposed between an inverter output and armature winding of a motor, directly detecting a driving current as a drop voltage in the resistor. Alternatively, the detecting circuit detects, with a resistor, a driving current captured by a current transformer as voltage.
In directly detecting a motor driving current with the resistor, when an inserted resistance is represented as R, a motor driving current is represented as I, and an input voltage of the AD converter is represented as V, it is possible to detect the motor driving current from a relation of V=I×R (see Patent Document 1). The resistance R is selected based on a maximum current necessary for driving the motor and a range of voltage that can be input to the AD converter.
In indirectly detecting a motor driving current via the current transformer, when a turn ratio of the current transformer is represented as N, a resistance inserted on a secondary side of the current transformer is represented as R, a motor driving current is represented as I, and an input voltage of the AD converter is represented as V, it is possible to detect the motor driving current from a relation of V=I×N×R.
Patent Document 1: Japanese Patent Application Laid-open No. 2000-139091 (FIG. 14)